THE CHARACTERISTICS OF TECTONIC FISSURITY OF OIL-BEARING MIDDLE CAMBRIAN QUARTZITIC SANDSTONE SERIES FROM THE ŻARNOWIEC AREA

Thh development of vertical tectonic fissures cutting·oil-bearing quartzitic sandstones of the Middle Cambrian age from the Żarnowiec area (northern Poland) is discussed. Depending on the value of strike of a fissure measured in relation to strike of beds (L - to the left and P - to the right in relation to the dip direction), the vertical tectonic fissures are assigned to the following direction systems: 1 (fissures parallel to dip of beds), 2 (parallel to strike of beds), 3a (L - 60;°), 3b (L - 40°), 3c (L - 30°), 4a (P - 30°), 4b (P - 50°), 4c (P - 60°). The fissures most common here are those of the systems 2 and 4 (300 and 400 fissures per cubic meter of rock, respectively) whilst those of the systems 1 and 3 are somewhat less common (200 fissures per cubic meter of rock). Fissures of· the systems 3c and 4c occur in subordinate numbers (see Figs. 1, 2). It follows that the degree of fissurity equals 1000-1100 fissures per cubic meter of rock: for sandstones of reservoir series of the Zarnowiec oil field. The summative surface of these fissures from one cubic meter of rock ranges from 5 to 12 m2, which corresponds to fissure capacity equal 0.10-0.15% of porosity. These values do not refer to horizontal fissures from relaxation nor lithogenic fissures.

The most intense traces of hydrocarbons are found along vertical tectonic fissures of the system 2 (fissures parallel to strike of beds). The analysis of. the mode of intersecting of tectonic fissures representing different systems as well as of verticaI tectonic fissures with horizontal or oblique fissures or stylolites of the first or second generation made it possible to establish the time sequence of origin of these textures: 1 - microstylolites of the first generation; 2 - vertical fissures systems (1 + 2); 3 - oblique fissures, stylolites and slicolitic surfaces; 4 horizontal relaxation fissures; 5 - vertical fissures systems (3a + 4a, 3b + 4b); and 6 - macrostylolites of the second generation. Stylolites and relaxation fissures were discussed in other papers of the present author.